TELEPHONE TRANSMISSION OVER LONG CABLE CIRCUITS 71 



Where I' is the velocity in unit lengths per second, L is the inductance 

 in henries per unit length and C is the capacity in farads per unit 

 length, the unit of length for expressing velocity, inductance and 

 capacity being the same. 



The X.L.L. type of loading is best for the longer circuits, because 

 of the more nearly equal attenuation of currents of different fre- 

 quencies, its higher velocity of propagation which permits more 

 efficient operation of telephone repeaters, and also its comparative 

 freedom from transient effects, as will be explained in more detail 

 later. For the shorter circuits where these effects are not so important, 

 the M.H.L. type is satisfactory electrically and is therefore employed 

 since fewer repeaters are required owing to the lower attenuation. 



III. "Echoes" 



As is well known, whenever points of discontinuity or unbalance 

 occur in a telephone circuit, reflections of electrical energy take place. 

 If the circuit is long so that the time for transmission is appreciable 

 and if also the losses are not so great as to cause the reflected energy 

 to become inappreciably small before it reaches the ear of a listener, 

 echo effects will be experienced. While, in general, reflections take 

 place in any telephone circuit actual echoes are never appreciable 

 unless telephone repeaters are employed. In the case of circuits 

 with repeaters, the electrical length is usually great enough so that 

 an appreciable length of time is required for the voice currents to 

 travel to some discontinuity and back again. Furthermore, the 

 repeater gains keep the reflected voice currents large. 



It should be understood that the echo effects which are experienced 

 in long repeatered circuits are due to the same unbalances, which, 

 on shorter circuits, bring in trouble due to "singing", or distortion 

 of the voice waves due to "near-singing". On electrically long cir- 

 cuits, due to the comparatively great time lags involved, the echo 

 effects become of controlling importance. Consequently, it is, in 

 general, necessary on such circuits to work the repeaters at gains 

 well below those at which "singing" or distortion due to "near-sing- 

 ing" is experienced. 



The echo effects which occur in four-wire circuits will first be dis- 

 cussed, since the effects are simpler in this case than they are in the 

 case of a two-wire circuit. 



Figure 3-a shows a four-wire circuit in diagrammatic form, while 

 Figure 3-b shows the echoes which are caused by the unbalances at 

 the terminals. When someone at terminal A talks to a person at 



